The present invention relates to communications systems, and more particularly, to echo cancelation in a bi-directional communications link.
In many communications systems, for example landline and wireless telephone systems, voice signals are often transmitted between two system users via a bi-directional communications link. In such systems, speech of a near-end user is typically detected by a near-end microphone at one end of the communications link and then transmitted over the link to a far-end loudspeaker for reproduction and presentation to a far-end user. Conversely, speech of the far-end user is detected by a far-end microphone and then transmitted via the communications link to a near-end loudspeaker for reproduction and presentation to the near-end user. At either end of the communications link, loudspeaker output detected by a proximate microphone may be inadvertently transmitted back over the communications link, resulting in what may be unacceptably disruptive feedback, or echo, from a user perspective. Furthermore, if the round-trip loop gain is greater than unity at any audible frequency, then the system will tend to "howl" as is well known in the art.
Therefore, in order to avoid transmission of such undesirable echo signals, the microphone acoustic input should be isolated from loudspeaker output as much as possible. With a conventional telephone handset, in which the handset microphone is situated close to the user's mouth while the handset speaker essentially covers the user's ear, the requisite isolation is easily achieved. However, as the physical size of portable telephones has decreased, and as hands-free speaker-phones have become more popular, manufacturers have moved toward designs in which the acoustic path from the loudspeaker to the microphone is not blocked by the user's head or body. As a result, the need for more sophisticated echo suppression techniques has become paramount in modern systems.
The need is particularly pronounced in the case of hands-free automobile telephones, where the closed vehicular environment can cause multiple reflections of a loudspeaker signal to be coupled back to a high-gain hands-free microphone. Movement of the user in the vehicle and changes in the relative directions and strengths of the echo signals, for example as windows are opened and closed or as the user moves his head while driving, further complicate the task of echo suppression in the automobile environment. Additionally, more recently developed digital telephones process speech signals through vocoders which introduce significant signal delays and create non-linear signal distortions. As is well known, these prolonged delays tend to magnify the problem of signal echo from a user perspective, and the additional non-linear distortions can make echo suppression difficult once a speech signal has passed through a vocoder.
Traditionally, echo suppression has been accomplished using echo canceling circuits which employ adaptive filters to estimate and remove echo signals from a microphone output so that only near-end speech and noise are transmitted over the communications link. Such systems are described, for example, in U.S. Pat. No. 5,475,731, entitled "Echo-Canceling System and Method Using Echo Estimate to Modify Error Signal" and issued Dec. 12, 1995, and U.S. patent application Ser. No. 08/578,944, entitled "Gauging Convergence of Adaptive Filters" and filed Dec. 27, 1995, each of which is incorporated herein by reference. While the systems described in the cited documents are generally effective in canceling or suppressing echo signals, certain characteristics of those systems make them impractical in some contexts.
For example, as is described in detail below, such systems are not well suited for double-talk situations in which a near-end user and a far-end user are speaking simultaneously. Additionally, the above-mentioned systems are not well suited for situations in which ambient noise, for example road and traffic noise, is prevalent at a microphone input. Thus, there is a need for an improved echo suppression system.